One known boiling water nuclear reactor includes a drywell, or containment, a wetwell, a Gravity Driven Cooling System (GDCS) and a passive cooling containment system (PCCS). The drywell is designed to withstand pressure generated by a reactor pressure vessel (RPV) during reactor operation, and the PCCS is configured to limit the pressure within the containment to a pressure below a design pressure of the containment and to keep the RPV core substantially cool.
The GDCS is substantially isolated from the drywell and is an emergency source of low pressure reactor coolant used following a loss of coolant event in at least one known boiling water reactor (BWR). A typical GDCS includes pools of coolant positioned so that when coolant from the pools must be supplied to the RPV, the coolant flows, under gravity forces, through the GDCS coolant delivery system into the RPV. Under normal reactor operating conditions, however, coolant from the GDCS does not flow into the RPV.
A typical PCCS includes several condensers positioned in a PCCS pool of water. Each condenser includes an upper drum, a lower drum, and several heat exchanger tubes extending between the upper and lower drums. The upper drums are coupled to the drywell via a steam inlet passage, and steam generated within the containment flows from the upper drums and to the lower drums through the exchanger tubes. The steam is condensed into water and noncondensable gases, e.g., noncondensables, flow between the upper and lower drums. The condensed steam is drained from the lower drums and to the condensate drain tank via a condensate drain line.
The noncondensables are drained from the lower drums utilizing vent lines which extend from each lower drum and into the wetwell suppression pool. Noncondensables are discharged from the lower drums and into the suppression pool, and rise through the suppression pool to the wetwell air space.
The wetwell is separated from the containment by a wall having an opening therein. A vacuum breaker typically seals the opening and is movable between an open position and a closed position. The vacuum breaker is a check valve which allows the noncondensables and steam to pass from the wetwell to the drywell and substantially prevent a large differential pressure from developing between the wetwell and the drywell. Particularly, if pressure in the wetwell becomes sufficiently great compared to pressure in the drywell, the vacuum breaker opens and noncondensables and steam in the wetwell flow through the vacuum breaker and into the drywell to reduce the differential pressure.
If the vacuum breaker becomes stuck in the open position, it is possible for the differential pressure between the wetwell and the drywell to reduce too much. Particularly, it is possible for steam in the drywell to bypass the PCCS steam inlet passage and flow directly into the wetwell via the vacuum breaker, which is undesirable.
To prevent a vacuum breaker from sticking in the open position, it is known to utilize an isolation valve. However, isolation valves sometimes fail and thus cause the vacuum breaker to cease operating. In addition, isolation valves must often be monitored to ensure proper operation.
It would be desirable to provide a system which substantially prevents steam and noncondensables from flowing from the drywell and into the wetwell while the vacuum breaker is in the open position. It further would be desirable for such system to facilitate the maintenance of an acceptable drywell to wetwell pressure differential.